Abstract Melanoma is one of the deadliest skin cancers in the United States and unlike most other major cancer types, its prevalence is increasing rapidly. Despite having various treatment options including radiation therapy and chemotherapy, curative treatment for metastatic melanoma remains elusive. Immunotherapy has shown promise in inducing clinical responses in a small subset of patients. However, the overall survival rate of patients with metastatic melanoma is disappointing. Thus, it is important to identify factors which limit the efficacy of immunotherapy such as Ipilimumab. Our studies have demonstrated that exposure of pro-oxidative stressors including ultraviolet B-irradiation to human and mouse skin generates oxidized lipid mediators with platelet-activating factor (PAF) agonist activity. These UVB generated PAF agonists act via PAF-receptor (PAF-R) and mediate systemic immunosuppression via cyclooxygenase 2 (COX-2), immunosuppressive cytokine, interleukin 10 (IL-10) and cell type regulatory T cells (Tregs). Importantly, we have recently shown that UVB/PAF-R agonists mediated systemic immunosuppression augments the growth of murine B16F10 melanoma tumors in a PAF-R dependent manner. These effects were mediated via up-regulation of IL-10 and Tregs in the tumor microenvironment. Notably, anti-oxidants and depleting antibodies against IL-10 and Tregs (anti-CD25) blocked UVB/PAF-R agonists-mediated enhanced tumor growth, indicating the involvement of oxidatively generated PAF-R agonists and downstream IL-10 and Tregs in immune evasion of B16F10 tumors. Thus, pro-oxidative stressors may play an important role in inhibiting immune-mediated clearance of melanoma and therefore could potentially compromise the efficacy of melanoma immunotherapy, aimed at suppressing immune escape mechanisms. Notably, we now demonstrate that UVB-induces a PAF-R dependent up-regulation of the negative co-stimulatory T-cell receptors CTLA-4 and PD-1 in the tumor microenvironment, which have been demonstrated to suppress normal immune responses against tumor antigens. Importantly, we show that PAF-R agonists induced systemic immunosuppression is mediated via myeloid derived suppressor cells (MDSCs), an immunophenotype known to suppress host anti-tumor immunity against target antigens. The studies in the proposed grant attempt to take initiative in a direction which could have a major impact on treatment options for melanoma. The preclinical studies outlined below will (1) potentially provide novel strategies to increase the efficacy of anti-CTLA-4 and/or anti-PD-1 melanoma immunotherapy, (2) explore mechanisms of how pro-oxidative stressors can inhibit host anti-tumor immunity. In addition, clinical studies will allow direct assessment of whether therapeutic dose of UVB can generate adequate levels of immunosuppressive PAF-R agonists in patients undergoing phototherapy. These studies will help in determining the impact of PAF-R agonists on the disease outcomes in patients. These studies will take an advantage of various transgenic and knockout mouse models as a tool for our research.